The formation of carbon-carbon bonds has long been a challenge to synthetic chemists. One such C--C bond-making process that is both intellectually appealing and of potential practical value is to convert carbon monoxide into functionalized hydrocarbons by reaction chemistry in which the first step involves reductive coupling.
One reductive coupling example is the direct reduction of CO by alkali metals (see, Liebig, J. Ann. Chem. Pharm., 11, 182 (1834); Joannis, Hebd. Seances. A.C.R. Acad. Sci., 116, 1518 (1893); ibid., 158, 874 (1914); Pearson, Nature, 131, 166 (1933); Hackspill et al., Hebd. Seances. A.C.R. Acad. Sci., 206, 1818 (1938); Scott, Science, 115, 118 (1952); Weiss, et al., Helv. Chim. Acta., 46, 1121, (1963); Ibid., 47, 1415 (1964); Buchner, Helv. Chim. Acta, 46, 2111 (1963); Weiss, et al., Z. Anorg. Allgem. Chem., 330, 251 (1964); Weiss, et al., Chem. Ber., 98, 126 (1965); and Ellis, et al., J. Am. Chem. Soc., 103, 6100 (1981)).
Another such method is electrochmical reductive coupling (see, Silvestri, et al., Gazz. Chim. Ital., 102, 818 (1972); Silvestri, et al, Electrochim. Acta, 23, 413 (1978); Bockmair, et al., Z. Naturforsch. B.: Anorg. Chem., Org. Chem., 30B, 330 (1975); and Uribe, et al., J. Electroanal. Chem., 152, 173 (1983)).
These two techniques both yield the species M.sub.2 (C.sub.2 O.sub.2) and higher oligomers, from which, upon further redox or solvolysis reactions, glycolic acid derivatives, ethylene glycol, oxalic acid, hexahydroxybenzene, and aci-reductone polymers have been isolated. See, in addition, to the previous citatons, Buchner, Helv, Chim. Acta, 48, 1229 (1965); and Barber, Ph.D. Thesis, Chapter III, Massachusetts Institute of Technology, 1981.
Reductive coupling of carbonyl ligands in soluble transition, lanthanide, or actinide metal complexes has also been achieved. (See, for example, Manriquez, et al., J. Am. Chem. Soc., 100, 2716 (1978); Wolczanski, et al., Acc. Chem. Res., 13, 121 (1980); Barry, et al., J. Am. Chem. Soc., 104, 4712 (1982); Fagan, et al., J. Am. Chem. Soc., 103, 6959 (1981); Katahira, et al., Organometallics, 1, 1723 (1982); Gambarotta, et al., J. Am. Chem. Soc., 105, 7295 (1983); Evans, et al., J. Chem. Soc., Chem. Commun., 706 (1981); Evans, et al., J. Am. Chem. Soc., 107, 3728 (1985); Planalp, et al., J. Am. Chem. Soc., 105, 7774 (1983): Erker, et al., Angew. Chem. Int. Ed. Engl., 25, 364 (1986); and Arnold, et al., J. Am. Chem. Soc., 107, 6409 (1985)).
This approach affords better control of subsequent reaction products and stereochemistry. In the cases reported thus far, however, the oxygen atoms of the coupled ligand are coordinated to a metal atom.
In recent years Lippard and his coworkers have been investigating the reductive coupling of alkyl isocyanide ligands in seven-coordinate complexes of the form [M(CNR).sub.6 X].sup.+ (M=Mo and W; X=halide or cyanide). (See also, Lam, et al., J. Am. Chem. Soc., 99, 617 (1977); Corfield, et al., Inorg. Chem., 20, 922 (1981); Dewan, et al., Inorg. Chem., 20, 4069 (1981); Giandomenico, et al., J. Am. Chem. Soc., 104, 1263 (1982); Caravana, et al., Inorg. Chem., 21, 1860 (1982); and Hoffmann et al., J. Am. Chem. Soc., 105, 146 (1983)). Also under investigation have been complexes of the form [Mo(CNR).sub.4 (bpy)Cl].sup.+. (See, Warner et al., Organometallics, 5, 1716 (1986)).
These two complexes may be illustrated by the following two equations: ##STR2##
Similar coupling reactions occur for Nb, Ta, and Mo complexes where, in the products, the coupled isocyanide ligand, formally the RNCCNR.sup.2- dianion, bridges two metal centers. (See, for example, Cotton, et al., J. Am. Chem. Soc., 105, 3734 (1983); Cotton, et al., J. Am. Chem. Soc., 106, 6987 (1984); and Lenz, et al., Angew. Chem. Int. Ed. Engl., 23, 525 (1984)).
From these experimental studies and extended Huckel molecular orbital calculations (Hoffmann et al., J. Am. Chem. Soc., 105, 146 (1983)) Lippard and his coworkers have identified several factors that promote the isocyanide reductive coupling reaction. Specifically, it has been discovered that eqs. (1) and (2) are favored by the high coordination numbers of the transition metals, the use of linear or chelating ligands, proper orbital alignment during the coupling step, an electron rich metal center and the need for a Lewis acid to bind the heteroatoms of the coupled ligand.
Using these factors as a guideline, Lippard and his coworkers searched for, and have now identified, a system in which reductive coupling of two carbon containing ligands, e.g., the carbon monoxide ligands in the known seven-coordinate [M(CO).sub.2 (dmpe).sub.2 Cl] complexes, [M=Nb, Ta and dmpe=1,2-bis(dimethylphosphino)ethane] (see, Datta, et al., Inorg. Chem., 16, 1134 (1977)) can be readily achieved.